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    <script>

        //       算术运算  -  *   /   %   会转为数字进行运算，但是注意变量本身不会变


        var a = 'hi';
        var b = 'hello';
        // a和b都是字符串

        var c = a - b;
        // 只有数字才有算术运算   a和b会自动转化成数字  NaN - NaN = NaN
        console.log(c);   // NaN

        // 变量的值只会在赋值的时候发生改变
        console.log(a);

        
    </script>

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